1. Field of the Invention
The present invention relates to a method of manufacturing a gradient-index glass and, more particularly, to a method of manufacturing a large (large-diameter) body of a gradient-index glass effectively used as a general optical lens of a camera or a microscope or a pickup lens for an optical disk. 2. Description of the Related Art
A gradient-index glass of this type is currently manufactured by an ion exchange method based on mutual diffusion of monovalent cations (e.g., thallium and potassium) in solid glass. A body of gradient-index glass manufactured in this manner and having a diameter of 1 to 2 mm is used in a lens array of a copying machine or a microlens for optical communication.
Since a diffusion rate of ions in solid glass in the above manufacturing method is very low, however, a large-diameter product for use in an optical system of a camera or the like cannot be manufactured.
For this reason, Published Unexamined Japanese Patent Application No. 63-2775 discloses a method of manufacturing a gradient-index glass in accordance with a sol-gel method using a silicon alkoxide as a main starting material as will be described below. First, a silicon alkoxide and a water-soluble salt (lead acetate) are used as starting materials to prepare a gel. Subsequently, the formed gel is aged, and water is removed from the gel. In this process, since the lead acetate in the gel is diffused toward the surface, this lead acetate in the gel is redissolved by a water-isopropanol solution mixture of lead acetate to cancel a concentration distribution caused by the diffusion, thereby obtaining a predetermined concentration. Subsequently, the resultant gel was treated by isopropanol to extract water in the gel. After these pretreatment steps, the gel is dipped in an aqueous potassium nitrate solution to externally diffuse and flow the lead acetate in pores of the gel, thereby obtaining a concentration distribution of the lead acetate. At the same time, potassium nitrate is diffused in the pore of the gel to compensate for a thermal expansion difference in the gel caused by the lead acetate concentration distribution. Subsequently, the gel is sequentially treated by using an isopropanol-solution and acetone to precipitate and fix the lead acetate and the potassium nitrate on the pore walls of the gel. Thereafter, the gel is dried in the air at room temperature and sintered to manufacture a gradient-index glass. Such a gradient-index glass manufacturing method, however, has the following problems.
That is, in the above manufacturing method, lead nitrate is added independently of lead acetate in order to obtain a homogeneous gel not containing bubbles by adjusting a reaction time. Since lead nitrate has a high solubility in a normal solvent, however, it is difficult to fix a concentration distribution of the lead.
In addition, in the pretreatment step, water required in gel formation is partially removed by evaporation in the air so as to easily fix the lead concentration distribution in the subsequent step. For this reason, the gel containing water deforms or cracks by its own weight in the water removal step. Furthermore, in the lead acetate concentration distribution formation step in which potassium nitrate is diffused in the pores of the gel to compensate for the thermal expansion difference in the gel caused by the lead acetate concentration distribution, it is difficult to obtain a concentration distribution of potassium nitrate opposite to that of lead acetate. For this reason, since the thermal expansion difference caused by the lead acetate concentration distribution cannot be canceled by potassium nitrate upon sintering, the gel may crack. This tendency occurs more conspicuously as the size of the gel is increased. As a result, the size of a glass body which can be manufactured is limited, i.e., a lens having a diameter of about 7 mm or more and a height of about 10 mm or more cannot be manufactured.
Moreover, in the lead acetate concentration formation step, lead acetate is present in the form of ions in the pores of the gel. When such a gel is dipped in an aqueous potassium nitrate solution, since the lead acetate flows out in the aqueous potassium nitrate solution within a short time, it is difficult to control the lead acetate concentration distribution. As a result, this difficulty and the difficulty in fixing the lead acetate concentration distribution described above make it impossible to manufacture a glass having a predetermined refractive index distribution.